Internal gear is usually a gear with its teeth cut in the internal surface area of a cylinder and meshes with spur gears.
In its manufacturing, due to its shape, the most common hobbing machine found in spur gear production cannot be used. Generally it really is made with gear shaper (or gear shaping machine) equipped with a 
pinion cutter. Recently, the efficiency of inner gear cutting provides been improved by a different procedure called skiving.
Because of the thinness of the ring, it is sometimes difficult to create high precision. However, gear grinding to boost precision grade is limited by the number of producers and their offered grinding facilities leading to very high cost.
Also, although it is generally too expensive to make helical teeth internal gears, Ever-Power has made it possible to produce them by adopting equipment that can cut them simply by making pinion cutters rather than needing helical guides.
Normally, internal gears can be used with the same module spur gears, depending on the amount of teeth combinations, nonetheless it is necessary to note the possibilities to getting involute, trochoid or trimming interferences. For using Ever-Power standard stock gears, please consult the PDF formatted “Specialized Info of Internal Gears” section for usable quantity of teeth combinations.
Internal gears are often found in applications involving planetary gear drives and gear couplings. There are three major types of planetary gear mechanisms: planetary, solar and star types. With respect to the type and the pattern of which shafts become input and result, many variations of velocity transmitting ratios and rotational directions are produced.
